In the FA field, driving control of a machine tool is performed such that the movement of a table or a tool for fixing a workpiece (work) follows a command. Control for driving a machine such that a tool position (i.e., the relative position of the tool to the work) accurately follows a command route (a command track) is called tracking control (or contour motion control). The tracking control is precisely performed using a numerical control apparatus or a servo control apparatus attached to the numerical control apparatus. The control target machine tool includes a plurality of shafts, each shaft respectively driven by a servomotor. The individual servomotors are respectively driven using servo control apparatuses.
Friction, backlash, and lost motion present in a machine system disturb the tracking control. When the moving direction of a feed shaft is reversed, the directions in which these disturbances act are reversed. The influence of the reversal of the directions conspicuously appears in tracking errors. A typical error, which occurs when an arcuate track is being followed, is a tracking error that occurs when the moving direction of the feed shaft is reversed in a quadrant switching portion of the arcuate track. The tracking error is called quadrant projection because a track has a shape projecting as a protrusion to the outer side when the error amount in the radial direction is enlarged and plotted. The occurrence of the tracking error of the track like the quadrant projection during machining is undesirable because streaks and flaws occur in the machining result.
Therefore, for example, a virtual feeding mechanism is incorporated and a correction command is calculated from the difference in the torque command signal between the virtual feeding mechanism and the actual feeding mechanism, and this is added to an actual torque command or frictional force or frictional torque that occurs before and after reversal of the motion direction of the feeding driving mechanism is estimated; a motor torque error is extracted; and torque compensation is performed according to an error signal (e.g., Patent Literature 1).